1. Field of the Invention
The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser having an active layer for constricting a current in the transverse direction of a cross-sectionally triangular body to reduce a threshold current, and a distributed feedback semiconductor laser of the above structure which is capable of single-mode oscillation.
2. Description of the Prior Art
SDH (Separated Double Hetero Junction) semiconductor lasers having low threshold currents Ith that can be fabricated in one epitaxial growth cycle have been proposed by the applicant as disclosed in Japanese laid-open patent publications Nos. 61-183987 and 2-174287, for example.
One such proposed SDH semiconductor laser is shown in FIG. 1 of the accompanying drawings. As shown in FIG. 1, an N-type substrate 1 made of GaAs, for example, has a principal surface as the {100} plane defined according to Miller indices. The substrate 1 is of a mesa structure having, on the principal surface, a stripe raised region or land 2 extending in the direction of the &lt;011&gt; axis normal to the sheet of FIG. 1. On the principal surface of the substrate 1, there are successively deposited, for example, an N-type buffer layer 13, a first N-type cladding layer 4, an active layer 5 which is of a low impurity concentration or is undoped, a second P-type cladding layer 6, an N-type current blocking layer 8, a third P-type cladding layer 9, and a P-type cap layer 10. These semiconductor layers are deposited in one epitaxial growth cycle according to the ordinary MOCVD (Metal Organic Chemical Vapor Deposition) process, i.e., the methyl MOCVD process.
The first, second, and third cladding layers 4, 6, 9 and the current blocking layer 8 are made of a material having a large band gap, i.e., a small refractive index, compared with the material of the active layer 5.
The crystal orientations of the substrate 1 and the strip raised region 2, the width of the raised region 2, the height of the raised region 2, i.e., the depth of grooves 2A on opposite sides of the raised region 2, and the thicknesses of the first cladding layer 4, the active layer 5, and the second cladding layer 6, are selected such that the first cladding layer 4, the active layer 5, and the second cladding layer 6 on the raised region 2 are separated from the layers over the grooves 2A by slant surfaces 7. The first cladding layer 4, the active layer 5, and the second cladding layer 6 on the raised region 2 jointly make up a cross-sectionally triangular stripe body 20 composed of epitaxial growth layers.
The cross-sectionally triangular stripe body 20 is formed because according to the ordinary MOVCD process that employs a gas of methyl organic metal, once the (111) B face is formed, epitaxial growth layers tend to develop at a reduced rate with respect to the (111) face. The current block layer 8 is divided by the cross-sectionally triangular stripe body 20 into two layer sections one on each side of the body 20. The divided layer sections of the current block layer 8 have opposite ends held against respective opposite ends of the active layer 5 in the body 20 which face the respective slant surfaces 7.
The active layer 5 in the body 20 on the raised region 2 is therefore sandwiched between the divided layer sections of the current blocking layer 8 whose refractive index is smaller than that of the active layer 5. The active layer 5 is therefore confined as a lasing region in the transverse direction of the cross-sectionally triangular body 20. Because the current blocking layer 8 is present, the third cladding layer 9, the current blocking layer 8, the second cladding layer 6, and the first cladding layer 4 jointly provide a P-N-P-N thyristor structure on each side of the cross-sectionally triangular body 20, for blocking a current through those layers. Accordingly, a current is concentrated on the active layer 5 in the body 20 on the raised region 2 for thereby reducing the threshold current.
The divided layer sections of the current block layer 8 have opposite ends held against and covering the respective opposite ends of the active layer 5 in the body 20 which are positioned at the respective slant surfaces 7. With such a layer configuration, some leak currents tend to flow through current paths, indicated by the arrows i.sub.1, that extend from the third cladding layer 9 through the N-type current blocking layer 8, the first N-type cladding layer 4, and the N-type buffer layer 13 on the raised region 2 to the N-type substrate 1.
To eliminate such current paths, there has been proposed another semiconductor laser as shown in FIG. 2 of the accompanying drawings. Those parts shown in FIG. 2 which correspond to those shown in FIG. 1 are denoted by identical reference characters, and will not be described in detail. In FIG. 2, the divided layer sections of the current block layer 8 have opposite ends held against respective opposite ends of the second P-type cladding layer 6 in the body 20 which face the respective slant surfaces 7. Therefore, the current block layer 8 does not cover the respective opposite ends of the active layer 5 in the body, and does not contact opposite sides of the first N-type cladding layer 4 which face the respective slant surfaces 7. With the structure shown in FIG. 2, inasmuch as no current paths i.sub.1 are produced, the leak currents are reduced.
In FIG. 2, current paths indicated by the arrows i.sub.2 extend from the third P-type cladding layer 9 through the second P-type cladding layer 6 in the body 20, the second P-type cladding layer 6 over the grooves 2A, and the N-type buffer layer 13 in the body 20 to the N-type substrate 1. However, since the lower surface of the current blocking layer 8 which is held in contact with the slant surfaces 7 is positioned closely to the upper surface of the active layer 5 in the body 20, the current paths defined between the lower surface of the current blocking layer 8 and the upper surface of the active layer 5 are very narrow, and any leak currents flowing through the current paths are small.
In the case where the active layer 5 is made of AlGaAs, leak currents described below will pose a problem. More specifically, current paths exist which extend from the second P-type cladding layer 6 to the upper surface of the raised region 2, i.e., to the N-type buffer layer 13, as indicated by the arrows i.sub.2, or to the N-type substrate 1. Since the built-in potential across the P-N junction in the current paths is lower than the built-in potential between the second cladding layer 6 and the active layer 5 on the raised region 2, leak currents flowing from the second P-type cladding layer 6 over the grooves 2A to the substrate 1 or the buffer layer 13 become dominant. As a result, the current is not sufficiently constricted by the active layer 5 in the transverse direction of the body 20, making it difficult to reduce the threshold level for the operating current.
Distributed feedback (DFB) semiconductor lasers are known as semiconductor lasers capable of single-wavelength oscillation. The DFB semiconductor lasers have a diffraction grating disposed near an active layer for oscillated emission with wavelength selective capability, i.e., lasing in a particular wavelength or a single longitudinal mode. Research and development efforts are being made to apply DFB semiconductor lasers to the wide-band transmission of optical signals over optical fibers.
Generally, when DFB semiconductor lasers are fabricated, the process of depositing semiconductor layers by way of epitaxial growth is interrupted by an etching process for forming the diffraction grating.
DFB semiconductor lasers are also of a structure for a low threshold current. When a strip active layer is embedded as a lasing region, the epitaxial growth process is interrupted and divided into two processes before and after an etching process for forming grooves, because of the embedded structure of the strip active layer, i.e., the structure for optical and carrier confinement.
Where interfaces divided by the two epitaxial growth processes are present in the vicinity of the active layer, the previously formed interface tends to be oxidized between the two epitaxial growth processes, resulting in a characteristic degradation.
SDH semiconductor lasers suffer another problem with respect to efforts to reduce the threshold current.
FIG. 3 of the accompanying drawings shows another conventional semiconductor laser. Those parts shown in FIG. 3 which correspond to those shown in FIG. 1 are denoted by identical reference characters, and will not be described in detail. In FIG. 3, the current blocking layer 8 grows along the {311} B plane, i.e., the (311) B face, in the vicinity of the cross-sectionally triangular body 20, and along the {100} plane, e g., the (100) face, remotely from the raised region 2. Therefore, regions that grow along a crystal face of higher order, i.e., crystal face regions 23 of higher order indicated by the broken line a, are developed between (311) B face regions 21 and (100) face regions 22.
In such a structure, it is necessary to reduce the thickness of the current block layer 8 so that it does not cover the second P-type cladding layer 6 over the raised region 2. If the width of the active layer 5 is reduced for a lower threshold current, then since the crest of the body 20 which is composed of the second cladding layer 6 is also reduced in size, the current block layer 8 should be further reduced in thickness.
The (311) B face region 21 tends to turn into an N-type region, and the crystal face region 23 where the (311) B face regions 21 progressively change to the (100) B face regions 22 tend to turn into a P-type region. Consequently, if the thickness of the current blocking layer 8 is 3000.ANG. or smaller, then a portion of the current blocking layer 8 turns into a P-type region in each of the crystal face region 23, or a substantially N-type region has a thickness of 1000.ANG. or smaller, producing a tunnel current. When leak currents are produced in the vicinity of the crystal face regions 23 of the current blocking layer 8, these leak currents flowing near the crystal face regions 23 become dominant, with a resultant less current flowing to the active layer 5. Accordingly, the P-N-P-N thyristor structure, referred to above, is virtually impaired, and the threshold current is increased.
One solution is to increase the concentration of the N-type impurity in the current blocking layer 8 to prevent the crystal face regions 23 from turning into P-type regions. However, such an attempt is apt to increase the thickness of the (311) face regions 21. It is thus necessary to control the (311) B face regions 21 such that they will not cover the second P-type cladding layer 6 over the raised region 2. As a consequence, the positions of the semiconductor layers cannot be selected with much leeway, resulting in a reliability problem of the semiconductor lasers.